Within a matter of years, they say, they will need to take costly steps to preserve the life of the 800-mile-long line.

If they aren't successful, ice and wax could become a serious problem for the pipeline, increasing the risk of corrosion and spills.

Alyeska Pipeline Service Co.'s sense of urgency isn't because the North Slope is running out of oil.

The Slope's producing oil fields still contain enough oil to supply the pipeline for at least several more decades. Many other oil prospects on land and in the ocean remain unexplored.

So what's the problem?

In the 1980s, at peak oil flows, a barrel of oil made the trip from Prudhoe Bay to Valdez in four days.

Now it takes 13 days.

The slower flow causes the temperature of the hot oil to cool faster. At some point, the oil temperature will dip below the freezing point of water along certain segments, unless Alyeska reheats the oil inside the pipe.

As it gets colder, ice and wax may coat the insides of the pipeline. The colder oil might also increase the risk of buried segments of the pipeline jacking up in the ground, company officials said. The problems have been building for decades and will only become more pressing as oil production declines further.

For example, Alyeska, owned by BP, Conoco Phillips, Exxon Mobil and two smaller companies, used to launch devices to scrape wax - a component of the oil - out of the pipe's interior every several weeks.

Now it's every four to seven days.

While ice formation is not yet a problem in the trans-Alaska pipeline, it was the alleged cause of Prudhoe Bay's second-largest oil spill from a smaller pipeline a month ago.

Alyeska officials said they don't know yet how soon they will need to make major upgrades to the trans-Alaska pipeline to deal with the colder oil temperature and how much it will cost. They hope to have some answers by the end of next year, when they conclude a $10 million study of the problem.

One thing they do know: New oil production from undeveloped oil prospects in the Arctic will not come on line soon enough to sidestep the problem.

A damaged trans-Alaska pipeline would be big trouble for the state. The millions of barrels of oil that flow through it every week supply most of the tax revenue in the state's general fund and 10 percent of the country's oil production.

It would be devastating for Alaska's private sector, which depends on the sustained flow of oil for thousands of jobs.

The health of the pipeline is also critical to BP, Conoco and Exxon, which pour their North Slope oil output into it. They've funded millions for corrosion-related repairs and other upgrades to the pipeline since it was built in the 1970s. They recently funded the $10 million project to study the reduced oil flow problem.

Alyeska has talked to the Joint Pipeline Office, a government agency, since the early 1990s about how declining oil flow could affect the pipeline, said Jerry Brossia, the agency's top federal regulator.

"It's always been on the radar screen," Brossia said. The Joint Pipeline Office is composed of a dozen state and federal agencies that regulate the trans-Alaska pipeline.

Among Alyeska's earlier projects to adapt to declining oil flow was its $500 million project, finished this year, to replace the pumps that move the oil to Valdez. That project was plagued with cost overruns and other mishaps. The new pumps are now configured to work when the pipeline is transporting as little as 300,000 barrels per day. This year, the pipeline moved roughly 700,000 barrels per day, one-third of its peak flow in the late 1980s. Alyeska officials said they can prevent reduced oil flow from harming the pipeline but they conceded their research is taking them into uncharted territory.

"There's very little information elsewhere in the world on running pipelines in the Arctic at low temperatures," said Mike Joynor, an Alyeska vice president involved in the $10 million study.

The company's inquiries led to a series of experiments that began last summer at an Imperial Oil-owned laboratory in Sarnia, Ontario, just north of Detroit. Imperial is a Canadian company partly owned by Exxon Mobil. Exxon has a 20.3 percent stake in the Trans Alaska Pipeline System, or TAPS.

The tests in Ontario involve running a couple different blends of North Slope crude oil through a series of pipe loops. In one loop, researchers working for Alyeska test the impact of cooling temperatures on wax suspended in the oil. In another, they test ice formation.

The pipeline company is also running tests on crude oil at its offices in Valdez and at the pipeline pump station near Delta, said Alyeska spokeswoman Michelle Egan.

Cooler oil is already causing headaches for the company.

Oil enters the pipeline at Prudhoe Bay at 110 degrees, but because it flows slower than in the past, the temperature drops quicker and remains below 70 degrees for much of its trip. When the oil temperature falls below 70, its wax content falls out. The wax will coat the pipe walls and hide corrosion if it isn't scraped out soon. Also, the wax can clog sensors on the large devices called smart pigs that shoot through the pipeline hunting for corrosion.

Alyeska got a wake-up call on wax in 2006, during the partial shutdown of the Prudhoe Bay field after a major oil spill. The sensors on a smart pig launched into the pipeline became clogged with wax and it failed to collect data. Since then, Alyeska has been sending scraper pigs, another bullet-shaped device, down the pipe on a weekly basis.

Alyeska officials believe they may see problems more worrisome than wax as soon as five years from now.

They say that when the flow rate drops to only 500,000 barrels of oil a day, the oil temperature at certain points along the route north of Fairbanks could dip below 32 degrees. The small amount of water suspended in the oil could settle out and form ice crystals. Ice that coats pipe walls could create a hospitable spot for corrosion. Ice chunks that could form might batter pump-station equipment, regulators worry. Also, during a long pipeline shutdown, ice could plug sections of the line, making it difficult to restart the oil flow.

Another problem might appear 15 to 20 years from now when the pipeline is projected to be moving just 300,000 or 350,000 barrels per day. At that point, the oil would no longer heat the ground around the buried sections of the pipeline enough to prevent the pipe from jacking up during frost heaves. While that is not expected to be a problem along the entire 400 miles of buried pipe, it would create too much strain on the pipe in certain areas, according to the company.

Alyeska officials said they are optimistic that all of the bad consequences of lower oil flows can be avoided.

The company is considering fixes such as heating up the oil, adding chemicals to prevent water from freezing or wax from clogging the pipeline, and removing the water suspended in the oil before the oil enters the pipeline, Joynor said.

The fixes will likely be costly, but they are "at the heart of preserving the viability of TAPS," he said.

If the company wants to proceed with any of those changes, it would need to get approval from state and federal regulators at the Joint Pipeline Office.

Alyeska's scrutiny of the problem comes at a time when oil prices remain high, but despite the high prices BP and Conoco - two of the majority owners of the pipeline - are cutting jobs and spending. They say they are suffering from higher state taxes, higher maintenance costs and greater resistance to Arctic oil and gas exploration.

Alyeska, the pipeline's operator, isn't immune from the cutbacks. Next year, the company says, it will trim its work force by 60 employees, replace some of its contractors with cheaper, non-union workers and slash its spending in Alaska by 14 percent.

Some pipeline watchdog groups are worried that the loss of experienced workers could increase the potential for spills or other accidents. Those things have happened during previous industry cutbacks, said Stan Jones, spokesman for the Prince William Sound Citizens' Advisory Council, a watchdog group that monitors the oil-laden tankers that leave Alyeska's port in Valdez.

The pipeline company said that despite the cutbacks it is doing more to preserve the line, hiring more staff who work on pipeline integrity issues and funding new projects to address corrosion.

Due to the uncertainty about oil prices and the North Slope's production decline, it's hard to predict when the oil companies will decide it no longer makes sense to run the pipeline.

The increased cost of piping the oil to the Valdez tanker port is just one part of their decision-making. Ultimately, the companies will determine whether the oil fields are generating the financial returns they want, state and federal officials said.